Multi-roll levelers are used as finishing tools for leveling steel sheet. The general principle of leveling by multi-roll levelers, in particular of tension leveling, consists in making the sheet or strip to be leveled pass between two series of parallel rolls arranged so as to be mutually imbricated, the imbrication decreasing in the direction in which the sheet runs. As it passes between the rolls, it is deformed in bending alternately in one direction and then the other. The amplitude of bending decreases from the entrance of the leveler to the exit, so that the steel strip is subjected to a succession of alternating stresses suitable for eliminating or at least greatly reducing the internal stresses that cause flatness defects. The progressive reduction in the deformation amplitude makes it possible to obtain, at the exit from the leveler, a strip as flat as possible and with as few internal stresses as possible. In tension levelers, the strip is driven through the leveler between a pay-out reel and a take-up reel by “S-shaped” drive units which make the strip run and also tension it.
The ever tighter tolerances, in terms of flatness and internal stresses, imposed by strip or sheet users mean searching for the best possible way of controlling the operation of levelers, with preadjustments being carried out, and the best possible understanding of the mechanical characteristics of the machine: play, clearances, spring, adjustment parameters, etc.
To gain a better understanding of the problems involved in achieving the desired improvement in the control of the behavior of levelers, the reader will be reminded of the principal components of a multi-roll leveler in relation to FIGS. 1 to 5.
The drawing in FIG. 1 shows schematically such a leveler, which comprises a set of lower rolls 11 and a set of upper rolls 12, supported by a lower beam 13 and an upper beam 14 respectively. The metal strip 10 runs through the leveler between two motor-driven units 31, 32 of drive and tensioning drums arranged in an “S-shaped” configuration in the direction of the arrow F. The rolls are all parallel and offset between the top and the bottom, in the running direction of the strip, so that they can be mutually imbricated to a greater or lesser extent. As may be clearly seen, in the entry zone of the leveler, the strip is relatively highly deformed by undergoing alternating bending between the entry rolls 11a, 12a, 11b, etc., which are highly imbricated, whereas in the exit zone the deformations are very slight because the exit rolls 11m, 12m, 11n are only slightly imbricated or not at all.
The drawing in FIG. 2 also shows schematically an example of means for adjusting the leveler, in order to adjust the imbrication of the rolls. The upper beam 14 is held on an upper frame 15 by adjustments assemblies 16a, 16b, 16c, 16d, for example of the type consisting of a screw-nut with angle gear, two assemblies 16a, 16b being placed near the entry of the leveler and the other two 16c, 16d being placed near the exit respectively, and on each side in the longitudinal direction. The two entry adjustment assemblies 16a, 16b are connected by a drive shaft 17a and a coupling 18a and are driven together by an entry motor 19a. Likewise, the two exit adjustment assemblies 16c, 16d are connected by a drive shaft 17b and a coupling 18b and are driven together by an exit motor 19b. 
The couplings 18a, 18b are used to temporarily uncouple the adjustment assemblies that they connect, in order to be able to adjust the transverse parallelism, or “dislocation”, between the lower and upper rolls, and to do so both at the entry and the exit of the leveler. Next, the imbrications of the rolls of the leveler are adjusted by means of motors which drive, simultaneously and in an identical manner, the adjustment assemblies, either at the entry of the leveler or at the exit.
The parallelism or dislocation adjustment is carried out only in the case of major interventions on the leveler. Calibration of the leveler is carried out more frequently, in order to readjust the imbrications of the rolls or to modify them according to the characteristics of the strips to be leveled.
FIG. 3 shows, also schematically, the leveler, seen from the front, in order to show the means for adjusting the bending or the crown of the rolls. This is because, during leveling, the bending forces exerted on the strip result, in reaction, in deforming the leveling rolls. To compensate for such deformation and prevent it in return from causing geometrical defects in the strip, the leveling rolls are in fact supported by back-up rolls that are themselves supported by press rolls. This assembly is mounted in a frame called a cassette placed on a set of tapered wedges or of actuators or else against supports that are independent and height adjustable, these being distributed over the width of the leveler. In the example shown in FIG. 3, there are eleven rows of press rolls 21 placed over the width of the leveler. The vertical position of the press rolls may be adjusted by means of adjustable tapered wedges 22, each acting under all the press rolls located on the same line parallel to the running direction of the strip and over the entire length of the leveler. The shape of the leveling rolls therefore depends on the vertical position of the press rolls.
An example of an adjustable press-roll system is shown in FIG. 5. In this example, the height of the press rolls is adjustable by means of tapered wedges 23 which are interposed between the support rolls and a rigid lower frame 15′ and which slide one over the other. The relative displacement of the tapered wedges is effected by a cylinder 24, and may be measured, for example, by a position sensor 25.
In the case of FIG. 3 for example, such systems have three press rolls 22a, 22b, 22c and 22i, 22j, 22k located on each side near the ends of the rolls, where the deformations are greatest. In the central part, it is unnecessary to use such adjustable press rolls. As may be seen in FIG. 4 in a highly exaggerated manner, the press rolls make it possible, by exerting under the rolls a vertical force of greater or lesser magnitude, to deform the latter, when empty and also under load, so that, during leveling, their profile is suitable for correcting the defects observed on the strip to be leveled.
To effect the overall adjustment of a leveler, there is therefore:                adjustment of the parallelism, or dislocation, of the leveler that is to say substantially the adjustment of the parallelism between the lower rolls and the upper rolls, this adjustment being carried out by acting on the screws for adjusting the position of the upper beam, taken independently between the adjustment screws on the right side and on the left side, after separating the couplings 18a, 18b;         adjustment of the imbrication of the rolls, at the entry and at the exit of the leveler, the amount of imbrication being as a general rule monitored by measuring the angle of rotation of the screws for adjusting the position of the upper beam, or by displacement sensors between the beams at the leveler entry and exit;        adjustment of the crown of the rolls by means of actuators, as described above, the value for each press roll being determined by the measurement made by the sensors 25;        tension in the strip, generated by the “S-shaped” tensioning units, the value of the tension being measured by a tensometer or from the electrical parameters of the reel motors; and        elongation generated during leveling and measured by a speed differential between entry tensioner and exit tensioner.        
Moreover, the precise geometry of the pass path of the strip, on which the quality of the leveling depends, itself depends on the forces generated during the pass and on the deformations of the strip, which forces and deformations cause deformations of the machine, called spring (or deflection or camber, cédage in French).
To be able to exert effective control of the leveling, it is necessary to know as precisely as possible the actual position of the leveling rolls and their geometry permanently during operation. It is therefore necessary to be able to determine what the geometry and the position of the rolls are according to all the other parameters that can have an influence on the rolls, that is to say the settings given to the various actuators and also the forces generated, that are likely to modify the geometry and the actual position of the rolls.
To be able, with full knowledge of the situation, to adjust the leveler according to the characteristics of the strip to be leveled and to be able to set the actuators, especially the motors for adjusting the imbrications, it is therefore necessary to calibrate or set the leveler, that is to say to determine the base adjustments of the leveler that are suitable for obtaining the desired leveling.
It is also highly desirable to be able to establish a relationship between the adjustment values that are controlled by the available actuators and the geometrical modifications of the leveling path during operation, in other words to know the spring of the leveler and to take this into account in the adjustment of the imbrication motors, in order as it were to compensate in advance for the spring that will be experienced during actual working.
Currently, the calibration of multi-roll levelers is conventionally carried out under no load using ground steel spacers, or under load using metal spacers and lead bars, that are slid between the beams of the leveler, then the beams are closed up in a parallel fashion until a precise gap is obtained between the two sets of lower and upper rolls, said gap being defined by a ground steel spacer, for example 8 mm in thickness, placed between the two sets of rolls. The leveler is thus placed under loading conditions suitable for compensating for the inevitable mechanical play and in a stress state defined by the compression of the lead spacers. The position of the leveler tightening screws in this state is then noted, said position then being taken as reference, with respect to which the subsequent working position is adjusted by bringing said adjustment screws into the position corresponding to the desired working position of the rolls and being based on the relationship linking the displacement of said screws to the corresponding displacement of the rolls. It is also possible to adjust the parallelism using this method, or to compensate for the dislocation, and even possibly to act on the press rolls in order to adapt the bending or the crown of the rolls, but only in an approximate manner.
During the abovementioned calibration procedure, the actual force undergone by the beams, due to the compression of the lead spacers, remains unknown and is therefore not reliably representative of the forces encountered during actual working.
In particular, the known method has the consequence, observed experimentally, of causing overtightening during the leveling of thin strip.
As a result, the residual curvatures—bow and crown—are not controlled satisfactorily.